1. Field of the Invention
The present invention relates generally to the field of electronic circuits, and more particularly to an apparatus and method for automatic control of amplifier gain using degeneration.
2. Description of the Related Art
In the integrated circuit designs, due to manufacturing process variations and the sensitivity of device parameters to temperature and supply voltage, the gain of amplifier stages is not well controlled. The variation of gain, or transconductance, of the amplifiers may be as high as a factor of 2 or more, especially when using a CMOS process. This large gain variation imposes difficulties for both circuit and system design in many applications.
Previously, this problem has been addressed by sensing the gain of the amplifier and using various feedback schemes to adjust the biasing current of the gain stage to minimize the variation in each amplifier. See for example, the article by Loinaz, published in the IEEE Journal of Solid State Electronics, December 1995, pgs. 1339-1349. By adjusting the biasing current for each amplifier, the gain may be normalized across each part.
In order to try to keep the gain constant across process and temperature variations, however, the biasing current will vary across a very wide range. This is especially true in deep sub-micron CMOS processes. Due to the velocity saturation, changes in the biasing current will not have enough impact on the gain. This may result in the biasing current moving from one limit to the other, causing the amplifier circuit to go out of the linear region or even not working.
Another approach to this problem has been to use a closed-loop feedback amplifier to reduce the gain variation. Closed loop feedback amplifiers have many drawbacks, however, and cannot be used in certain applications. A big problem is that these circuits are slow, so they cannot be used for high-speed applications, such as RF gain stages. Additionally, for high-speed applications such as RF gain stages, closed loop amplifiers are potentially unstable.
Thus, there is a need from an improved technique to control amplifier gain across temperature and process variations.